Did bubonic plague cause the Little Ice Age?

No one’s sure what caused the Little Ice Age, when global temperatures dipped by about 1 degree Fahrenheit — though it was much colder in Europe — between about 1550 and 1850.

The Thames River regularly froze over and the British held “frost fairs” on the ice. In the winter of 1780, New York Harbor froze, allowing people to walk from Manhattan to Staten Island. And Eskimos landed their kayaks in Scotland as the Arctic ice extended so far.

Many theories have been proposed to explain the chill: a decrease in solar activity, an increase in volcanic activity that would have reduced sunshine heating or a change in ocean circulation that might have cut off warm water from reaching northern Europe.

The team found an increase in cereal pollen from 1200 onwards (reflecting agricultural expansion), followed by a sudden dive around 1347, linked to the agricultural crisis caused by the arrival of the Black Death, most probably a bacterial disease spread by rat fleas.

This bubonic plague is said to have wiped out over a third of Europe’s population.

Counting stomata (pores) on ancient oak leaves provided van Hoof’s team with a measure of the fluctuations in atmospheric carbon dioxide for the same period. This is because leaves absorb carbon dioxide through their stomata, and their density varies as carbon dioxide goes up and down.

“Between AD 1200 to 1300, we see a decrease in stomata and a sharp rise in atmospheric carbon dioxide, due to deforestation we think,” says Dr van Hoof, whose findings are published in the journal Palaeogeography, Palaeoclimatology, Palaeoecology.

But after AD 1350, the team found the pattern reversed, suggesting that atmospheric carbon dioxide fell, perhaps due to reforestation following the plague.

Not all historians agree with the new theory, of course.

But I find this fascinating for a couple of reasons:

1. I love it any time science can provide a new perspective, a new illumination of history, and 2. Understanding the Little Ice Age has serious implications for fully comprehending the forces driving modern climate change, and its implications for our society.

18 Responses

As a medical humanities student with a particular interest in the history of medicine and medieval medicine in particular, I’m a bit confused by your post. How does the study you cite present a causal explanation between the Black Death and the Little Ice Age? To be sure, to the extent the stomata levels change right about 1350, there seems a fascinating association between the prima pestilentia and the incipient Little Ice Age, but I don’t see much explanation of a causal mechanism there, apart from the reforestation hypothesis, which is historically dubious.

While the image of the empty fields and abandoned villages following the Black Death has been overstated, there is no doubt, as can be seen from the ample documentation of labor shortages and concomitant political disputes (culminating in the 1381 Peasants’ Revolt), that the possibility that significant reforestation occurred in the century or two after the Black Death seems extremely unlikely. The human resources to do so simply weren’t available.

In favor of the theory, however, is the notion that the plague forced all manner of technological innovations, precisely because of the shortage of labor. It is no accident that Gutenberg invented his press in about 1450 — the costs of producing manuscripts in the traditional medieval method had increased substantially because there simply were less qualified scribes available to do the tedious copying.

If agricultural innovations allowed more reforestation on the back end of the time period in question, perhaps the theory isn’t so far-fetched. Still, 200 years (1350-1550) is an awfully short time from a climatological perspective, isn’t it?

Anyway, don’t take this as an argument, really — more of a quibble from a history geek. One thing that becomes apparent from studying the Black Death, is how unsurprising it is when new theories come forth for the ways in which the plague helped to created the world we live in right now. Our society looks like it does because of, not in spite of, the Black Death.

Thanks for your thoughtful comments. I’m a history geek, too, and am really getting into Medieval history at the moment.

I agree that this idea seems a little suspect — there’s certainly nothing causal, it’s just a theory. And like you mentioned, the environmental recovery of forests seems too sudden of an effect to actually have caused the Little Ice Age. It certainly doesn’t explain why temperatures were much lower than during the rise of Medieval culture, when much of the deforestation took place in northern Europe.

My linking to the article wasn’t necessarily an endorsement of the science. My goal was to stimulate a discussion of the Little Ice Age, one of the greatest mysteries of recent history, and one which science has a legitmate chance of actually solving. (As opposed to, say, the controversy over whether Charlemagne really did want to be crowned Emperor by the Pope in 800).

No problem at all. I too find the Little Ice Age fascinating. There was a great program on it on History Channel over the weekend, I believe. As for the Charlemagne problem, let me know if you hear of anyone solving that one. 😉

I have no knowledge in these fields, just some questions: if reforestation caused a significant drop in temperature after the Plague due to a reduction in carbon dioxide, shouldn’t the original higher level of forestation before 1200, and the accompanying lower levels of carbon dioxide, have resulted in similarly cold temperatures before the agricultural expansion that began around 1200? And would it be possible to detect a “greenhouse effect” between 1200 and 1350 in biological records?

Also, would the absence of human labor necessarily have prevented significant reforestation in the aftermath of the Plague? Couldn’t forests regenerate on their own without human intervention, as we’ve seen right here in the United States in the 20th century, when formerly worn-out agricultural areas in Appalachia were converted to national parks? It seems to me that reforestation after 1350 would be more inevitable than not, with human interference reduced.

Your point on reforestation is well-taken, though I’d still suggest that the notion that such significant reforestation could occur in two centuries that would have a non-negligble causal effect on a climatological phenomenon over an entire continent is quite dubious.

Anyway, the researchers do little more than point to an association between stomata levels, the prima pestilentia, and the beginnings of a dramatic cooling period over the Eurasian land mass. They don’t really articulate much of a causal connection between the stomata, the plague, and the Little Ice Age, for good reason, I tend to think.

However, and I should have mentioned it earlier, the association between significant ecological disturbances and outbreaks of plague is quite persistent in the historical record. This is especially so during the years preceding the prima pestilentia. One of the most recent authors on the Black Death, John Kelly, argues that such change may have a pronounced effect on driving the animal vectors (rats, of course, and, according to Russian researchers, marmots) into closer proximity to humans.

The possibility of a kind of discursive effect — that an epidemic that may have been brought about, in part, by climatological changes, in turn produced climatological effects of its own — is quite fascinating, if sheer conjecture, at this point.

The much more logical correlation would be to say that the deteriorating weather caused by the cooling period left people more vulnerable to disease, and that since dead people don’t cut down many trees, the forests recovered for a period of time.

That theory is not conjecture, and it is not original to me. I have read it in more than one place. The climate change that caused the so-called ‘mini iceage’ also ended Greenland’s attractiveness for European development, ended the forests in Iowa/Nebraska, etc.

I am a bit surprised someone would make the opposite cause-and-effect assertion, frankly.

I’m still wondering about the vector theory. Assuming that deforestation and increased carbon dioxide from about 1200 to 1350 caused a rise in temperatures, wouldn’t that make rats less likely to invade people’s dwellings for warmth? (I assume the rats weren’t moving in because of the air conditioning.) So there might be less contact between vectors and humans in a time of warming, not more, reducing the transmission of plague, not increasing it.

If I’m making incorrect assumptions about the relationships between atmospheric CO2 and temperature, or about temperature and rodent behavior, please clarify.

Scott, the fleas were brought on rats from central asia, and moved in via trade routes. It is fairly well documented, if I recall. It is another reason why I think this Dutch ‘theory’ is BS and is yet another example of being able to say that all things observed are due to global warming. If the climate was so sensitive as to falter in a pre-industrial age due to plague, how could it possibly stand up to the assault of large volcanic eruptions or forest fires? The more I think on this Dutch study, the sillier it looks.

You’re correct. After the crusades, the West began intermingling far more with the East, including via sea travel. Correspondingly rats had many more opportunities to travel from Asia and the Middle East into Europe.

Thanks. The mini-ice age left populations more vulnerable due to falling drop yields lowering calorie intake and colder weather. Graveyard studies show increased infant mortality and poorer health in adults. Remember this is when Greenland stopped being so green. This is when the forests in the Iowa/Nebraska area went away and grasslands took over. There, too, studies of graves show more infant mortality and poorer health. This is another reason, as I have mentioned in other threads, that I am still a skeptic on the mankind-driven climate change claims. And frankly the claims of the Dutch study makes me more so.

There is evidence that the Little Ice Age was more regional than global, pointing to a mechanism similar to one we are currently concerned about, the cooling of Europe due to a weakening of curents in the Northern Atlantic. I won’t go into detail, but WHOI provides a good summary of the mechanisms involved.

This still begs the question of what caused the warming of the poles during this time, but it does seem to dismiss any effects of agriculture. One might argue that reforestation slightly prolonged the Little Ice Age, but there is no evidence of any causal relatonship.

Bad science reported prematurely may be interesting fodder for discussion, but it should be underscored for what it is, more speculative fiction than science.

In regard to deforestation and global warming, here’s a story from Canada where warming is preceding and facilitating deforestation, rather than being caused by deforestation. This is an AP report that I heard on the radio and found after much searching on DC radio station WTOP’s Website (gotta be careful about attribution!)

“The (mountain pine) beetles (which kill lodgepole pine trees) are native to British Columbia and the Inland Northwest, but warm winters and an abundance of lodgepole pine are helping the insects flourish, according to a 2005 report from the University of British Columbia’s Forest Resources Management Department.

Officials say the beetle outbreak is the worst natural disaster to ever befall British Columbia and a researcher at the University of British Columbia says the province has little choice but to salvage what it can (by harvesting massive numbers of infected trees that can still be used commercially).

The infestation is the worst on record, 20 times larger than in the 1930s when 1.2 million acres were killed. Experts are expecting the epidemic to last another 10 years, or until about 80 percent of British Columbia’s lodgepole pine forests are wiped out.”

So now we’re getting into chicken-or-the-egg arguments. Will we see a significant regional warming in Western Canada caused by this huge warming-facilitated deforestation covering an area 40% the size of Idaho? If so, does this suggest that global warming can become a self-perpetuating cycle, supporting arguments in earlier blogs that global warming may reach an irreversible tipping point in this century?

Now, here’s a question related to evolution. Isn’t it counter-intuitive for pests like mountain pine beetles to kill off their hosts, thereby depriving themselves of their own habitats? If they weren’t so “greedy”, couldn’t they just eat a bit of bark then move on? You could ask the same thing about bacteria and viruses that cause terminal illnesses. (Tom’s favorite liberal lefties would argue that humans do exactly the same thing by driving gas-guzzling SUVs, refusing to recycle, and eating Freedom Fries at the local Golden Arches.)

Scott, I would like to be around to see the future applications of the bio nanocomputers Eric recently posted about, and where you and I both speculated as to their possible uses for triggering biomechanisms.

The agricultural uses for these things could certainly involve efforts to reforest.

Imagine a forest that responds to an insect attack or other calamitous infestations by releasing an internal repellant harmless to the tree but noxious or even fatal to the attacking bugs, along with another biomolecular machine that stimulates repair of the trees.

As I’ve told Eric privately, I’m building a lab on Dickinson Bayou and am looking forward to practicing science full time when I retire. Maybe these little computers will give me something to experiment with in the lab! Got plenty of land and fauna around me there, and a bayou for samples for water work!

ttyler5, I can see the headline now: ‘Mad scientist releases nanobot ravagers into Galveston Bay’, lol.

I think we should be very careful about having nanobots acting as an immune system for a natural system or process. Nanos are giong to evolve in the wild, and programming them to kill off or suppress whole species of anything is something that even I, a member of the VRWC, would tiptoe very carefully into.

Hunter, I had something in mind too simple to evolve! Just a switch, and activated by remote! No AI, I’d be afraid we’d have Ents running about, and those old oaks and pines are dangerous enough already in a high wind! :^D And gawd only knows what is growing in the Bayou already! I don’t walk along the shore at night, now, unless I’m armed!

I’d be all in favor of anything that could safely reforest the Appalachians, which between acid rain, various blights, and repeated attacks by the remnants of hurricanes are starting to look pretty ragged, even in the national parks. A simple cold front-induced high wind event this past January shredded many of the remaining trees in Shenandoah National Park. The hemlocks have all died since 1995 because of a beetle, and the chestnut blight of the 1930s still lurks, killing new saplings after 10 years or so of growth. I often wonder if the whole place will just be mountain laurel and other shrubs in 10 more years.

There’s one advantage that they have in West Texas: it’s darn near impossible to kill a mesquite tree. I’ve seen a mesquite forest survive a direct hit by an F4 tornado in Wichita Falls. I grew up climbing on a pair in my front yard that had to be at least 100 years old, judging by their size and their complete lack of obvious visible growth in the past 30 years. If I’m not mistaken, the spread of mesquite forests in Texas is at least partially due to abuse of grasslands by cattle grazing that left the land denuded and receptive to the mesquite beans that the cattle themselves carried around in their digestive tracts–a classic example of a plant evolving to take maximum advantage of its environment.